1. Field of the Invention
The present invention relates to an inkjet printing apparatus and carriage control method. Particularly, the present invention relates to an inkjet printing apparatus and carriage control method for printing an image on a printing medium by moving a carriage to which a printhead is mounted.
2. Description of the Related Art
Printing apparatuses mounted in a printer, copying apparatus, facsimile apparatus, and the like are conventionally configured to print an image (including a character and symbol) on a printing medium such as paper or a plastic film (for example, an OHP sheet) based on image information. These printing apparatuses are classified by the printing method of a printing means for use into an inkjet type, wire dot type, thermo-sensitive type, heat transfer type, laser beam type, and the like.
Of these printing apparatuses, a printing apparatus of the inkjet type (to be referred to as an inkjet printing apparatus) prints by discharging ink from an inkjet printhead (to be referred to as a printhead) serving as a printing means onto a printing medium such as printing paper. The inkjet printing apparatus can advantageously downsize the printing means and quickly print a high-resolution image.
In some cases, stable ink discharge becomes difficult at only a specific nozzle out of a plurality of nozzles which are formed in a printhead and discharge ink. The specific nozzle is called a failure nozzle. The failure nozzle cannot print satisfactorily, and a portion having a print failure in a printed image causes print unevenness. As a result, stripe-like density unevenness appears on the printed image.
To detect such an ink discharge failure using an optical sensor, it is necessary to align the optical axis of light irradiated from the optical sensor with the nozzle array of the printhead and discharge ink from each nozzle so as to cross the light path. In this case, the moving amount of the printhead from a reference position (home position) to the optical axis is set in advance. The printhead is moved by this moving amount, aligning the optical axis of the optical sensor with the nozzle array of the printhead. For recent higher-resolution color printed images, the interval between a plurality of nozzle arrays arranged in a printhead for color printing is decreasing, and higher alignment precision is required.
There is also proposed a method of implementing quick, high-precision position control even in the presence of an individual difference in the friction between an alignment control target such as a carriage and a carriage movement mechanism and the difference in use environment (see, for example, Japanese Patent No. 3,658,340).
When the printhead discharges ink, mist-like small droplets (to be referred to as mist) are created in addition to ink droplets which form an image. The mist is attached to every portion within the printing apparatus. Depending on how much and where the mist attaches, the friction characteristic (static friction and kinetic friction) between the carriage guide rail and the carriage changes.
Japanese Patent No. 3,658,340 proposes a method of compensating for the static friction by specifying the initial value of an integral compensation amount calculated by integral processing in PI control generally used as a feedback control method.
However, the conventional method considers only the static friction. The integral compensation amount varies when the static friction increases such that it is much more than the kinetic friction. Especially in a high-precision alignment operation, persistent oscillations are generated near the target position. For this reason, the alignment and output often take time. In addition, the nozzle array of the printhead and the optical axis of the optical sensor sometimes do not match each other, and no ink discharge failure can be detected. This results in poor printed image quality.